Photonic crystal fiber assembly

ABSTRACT

The invention comprises a photonic crystal fiber (PCF) assembly comprising a PCF and at least one ferrule structure. The PCF has a center axis and comprises a core region and a cladding region and a first fiber end section with a first fiber end. The ferrule structure has a center axis and is mounted to the first fiber end section. The ferrule structure comprises an inner ferrule arrangement and an outer ferrule arrangement surrounding the first fiber end section. The inner ferrule arrangement comprises an inner ferrule front section proximally to the first fiber end and an inner ferrule rear section distally to the first fiber end, and each of the inner ferrule sections have an inner diameter and in at least a length thereof fully surrounds the PCF. The inner ferrule rear section is anchored in an anchor length section to the first fiber end section and the inner ferrule front section supports the first fiber end section proximally to the first fiber end.

TECHNICAL FIELD

The invention relates to a photonic crystal fiber (PCF) assemblycomprising a PCF and at least one ferrule structure. The invention alsocomprises a laser system and an apparatus comprising such laser system.Further the invention comprises a set of correlated ferrule elements.

BACKGROUND ART

Photonic crystal fibers, herein referred to as PCF, belong to a class offibers comprising optical nano or micro structures that affect themotion photons. PCF (sometimes called holey fiber, hole-assisted fiber,microstructure fiber, or microstructured fiber) at least partly obtainsits waveguide properties by an arrangement of microstructures e.g. inthe form of air (or gas) holes or solid microstructures with arefractive index differing from the surrounding background material.There is a great variety of hole/microstructure arrangements, leading toPCFs with very different properties.

Examples of PCFs include the PCFs described in U.S. Pat. No. 6,985,661,8,938,146 or 7,792,408.

The termination of such PCFs is often rather difficult in particular dueto the microstructures and optionally hollow microstructures and/or ahollow core.

Terminal structures for traditional step-index fibers, i.e. opticalfibers with a core having a uniform refractive index and a surroundingcladding having a lower refractive index providing a sharp decrease inrefractive index at the core-cladding interface, are well known. U.S.Pat. No. 4,737,011 discloses a connector for a large core step-indexfiber designed such that high power light pumped into the end of thefiber has low risk in burning or melting the connector material. Theconnector comprises a holder with a metallic plug body and a radiallyinwardly arranged sleeve having a support portion adapted to support theoptical fiber at a distance from the end-facet of the optical fiber andwherein the support portion employs a transparent or a translucent heatresistant inorganic substance e.g. ceramic material or sapphire having amelting point of 1500° C. or more and having a refractive index higherthan that of the cladding of the optical fiber. It is not described howthe fiber is mounted in the connector.

Generally it is desired to terminate an optical fiber such that it issimple to handle and is sufficiently protected against dust, moistureand heat.

The small diameter and core diameter of the PCF and its typically highflexibility require that a termination of the PCF is held in amechanically rigid structure—normally called a ferrule or a ferrulestructure—at termination points in order to be practically useful inprecise beam delivery systems.

U.S. Pat. No. 7,242,835 discloses a fiber termination combination whichincludes an optical fiber having a fiber core for transmitting a highlyenergetic optical signal that can damage the fiber, and a structuredregion around the core for directing the optical signal into the core,the structured region being characterized by multiple channels ofsmaller internal diameter than the core defined by thin walls disposedaround the core; a ferrule, with an opening therein for locating thefiber, at the end of the fiber enveloping the fiber extremity whichcooperates with the blocking structure to block the optical signal fromimpinging on the microstructured region of the fiber; and a blockingstructure disposed over the end of the fiber with an opening mating withthe fiber core, the blocking structure blocking the optical signal fromimpinging on the microstructured region of the fiber.

U.S. Pat. No. 7,373,062 discloses an optical fiber which comprises ahollow fiber core, wherein the front faces of both fiber ends of thehollow fiber core are open and each fiber end is surrounded by aprotection element in a dustproof fashion. The protection elementincludes a window at its front face in front of the fiber end to coupleand decouple light to and from the hollow fiber core.

U.S. Pat. No. 7,306,376 discloses a monolithic optical ferrule wherein afiber is terminated bonded by fusion to form a monolithic unit whichminimizes optical loss and is typically capable of transmitting highpower laser radiation, preferably in the order of 500 W and higher,without damage to the fiber and ferrule. The end cap, fiber and fusiblepowder are composed of material of substantially the same physicalcharacteristics such that, when all are fused together, the structure soformed is monolithic and the optical path is transparent.

The prior art fiber terminations disclosed above are generally difficultto mount to the fiber and often result in damaging of the fiber orresult in a poor alignment in the z-direction (the axial direction ofthe fiber) and/or a poor anchoring of the optical fiber which result ina poor coupling of the fiber to another element.

DESCRIPTION OF THE INVENTION

The object of the invention is to provide an end termination structureof a PCF which alleviates at least one of the drawbacks described above.

In an embodiment it is an object to provide an end termination structureof a PCF which provides a good alignment of the PCF and the ferrule inthe z-direction and which is relatively simple to assemble.

In an embodiment it is an object to provide an end termination structureof a PCF which provides a safe protection of the PCF, at its terminatedend, in particular against dust, moisture and/or heat.

In an embodiment it is an object to provide an end termination structureof a PCF which provides new options for stripping off cladding modes.

In an embodiment it is an object to provide an end termination structureof a PCF which has a long lifetime even where the PCF is operating atrelatively high power, such as power above 5 KW.

These and other objects have been solved by the invention or embodimentsthereof as defined in the claims and as described herein below.

It has been found that the invention or embodiments thereof have anumber of additional advantages which will be clear to the skilledperson from the following description.

The term “z-direction” means the axial direction and the term “axially”means along the axis.

The phrase “radial distance” means distance determined in radialdirection from the center axis of the structure in question, such as thePCF, the ferrule structure or an element thereof. The phrase “radialdirection” is a direction from the center axis and radially outwards and“radially” in a radial orientation relative to the axis.

The term “substantially” should herein be taken to mean that ordinaryproduct variances and tolerances are comprised within the scope of theterm.

The term “about” is generally used to include what is within measurementuncertainties. The term “about” when used in ranges, should herein betaken to mean that what is within measurement uncertainties is includedin the range.

It should be emphasized that the term “comprises/comprising” when usedherein is to be interpreted as an open term, i.e. it should be taken tospecify the presence of specifically stated feature(s), such aselement(s), unit(s), integer(s), step(s) component(s) and combination(s)thereof, but does not preclude the presence or addition of one or moreother stated features.

Throughout the description or claims, the singular encompasses theplural unless otherwise specified or required by the context.

All diameters are cross sectional diameters unless otherwise specified.

According to the invention an end termination structure has beenprovided in the form of a PCF assembly.

The PCF assembly of the invention comprises a PCF and at least oneferrule structure. The PCF has a center axis and comprises a core regionand a cladding region and a first fiber end section with a first fiberend.

The ferrule structure has a center axis and is mounted to the firstfiber end section. The ferrule structure comprises an inner ferrulearrangement and an outer ferrule arrangement surrounding the first fiberend section. In accordance with the invention it has been found that byproviding the ferrule structure with an inner ferrule arrangement and anouter ferrule arrangement a new type of the ferrule structure isprovided which provides a much simpler assembling of the fiber and theferrule structure. In particular it has been found that by shaping theinner ferrule arrangement such that is comprise an inner ferrule frontsection and an inner ferrule rear section, a much simpler assembling isprovided.

In an embodiment the inner ferrule arrangement comprises the innerferrule front section proximally to the first fiber end and the innerferrule rear section distally to the first fiber end. Each of the innerferrule sections has an inner cross sectional diameter and in at least alength thereof fully surrounds the PCF. The inner ferrule rear sectionis anchored in an anchor length section to the first fiber end sectionand the inner ferrule front section supports the first fiber end sectionproximally to the first fiber end. Advantageously the inner ferrulefront section is not fixed or anchored to the first fiber end section,but merely supports the first fiber end section to ensure a correctradial alignment.

In an embodiment the phrase “proximally to the first fiber end” meanclose to the first fiber end, preferably adjacent to the first fiber end

The PCF center axis at the first fiber end section and the ferrulestructure center axis are preferably substantially parallel. The PCF mayin principle be arranged at a distance from the ferrule structure centeraxis, however for most applications it is desired that the PCF centeraxis at the first fiber end section and the ferrule structure centeraxis are coincident, thereby making the assembling simpler.

In the assembling the first fiber end section is mounted and anchored inthe inner ferrule rear section. Subsequently, prior to or simultaneouslythe first fiber end section is mounted in the inner ferrule frontsection and the first fiber end is aligned to a desired position in thez-direction. Thereafter the outer ferrule arrangement is mounted to holdthe inner ferrule front section and the inner ferrule rear section inposition relative to each other and thereby to fix the first fiber endalignment relative to the ferrule structure. By providing the directanchoring of the fiber exclusively to the inner ferrule rear section amuch simpler alignment of the fiber end facet may be obtained since thedistance between the inner ferrule front section and the inner ferrulerear section may be adapted to position the fiber end facet at a desiredposition in z-direction before fixing the distance between the innerferrule front section and the inner ferrule rear section by mounting theinner ferrule front section and the inner ferrule rear section to theouter ferrule arrangement.

In an embodiment the inner ferrule front section and the inner ferrulerear section are not directly fixed to each other, but they are merelycoupled and held in position by the outer ferrule arrangement.Preferably the inner ferrule front section and the inner ferrule rearsection are arranged to have an intermediate gap in the axial direction.The gap advantageously provides a distance in the axial directionbetween the inner ferrule front section and the inner ferrule rearsection. The distance is preferably in the range from about 1 mm toabout 10 cm, such as from about 5 mm to about 2 cm.

In an embodiment the intermediate gap between the inner ferrule frontsection and the inner ferrule rear section extends partly around thePCF, for example to form a semi-annular gap. In an embodiment the gapextends fully around the PCF to form an annular gap and thereby adistance between the inner ferrule front section and the inner ferrulerear section. In both instances a part of the first fiber end section isnot covered by the inner ferrule arrangement.

In an embodiment the assembly further comprises an alignment sleevearranged between the inner ferrule front section and the first fiber endsection. Thereby the inner ferrule front section supports the firstfiber end section proximally to the first fiber end via the alignmentsleeve. The alignment sleeve is preferably arranged to surround andsupport the first fiber end section at the first fiber end.

In the assembling the alignment sleeve is mounted to the first fiber endsection in a desired position in z-direction relative to the first fiberend and thereafter the inner ferrule front section is mounted.

The alignment sleeve and the use thereof has been found to be verybeneficial because PCFs generally differ much in outer diameter andtherefore without an alignment sleeve would require individual sizes ofinner ferrule front sections. Further—as explained below—the alignmentsleeve may be provided to fit to PCFs with different outer diameterse.g. by crimping the alignment sleeve around the fiber to providesufficient support.

Advantageously the alignment sleeve supports the first fiber endsection, by being collapsed onto the first fiber end section in itswhole length or in the support section thereof. Suitable material forthe alignment sleeve therefore includes material which may be crimped orcollapsed onto the first fiber end section e.g. by application of heat.The alignment sleeve is advantageously not fixed or anchored to thefirst fiber end section but is preferably held mechanically in positionto provide a desired axial support to the fluorophores.

In an embodiment the alignment sleeve is fused to the PCF.

Advantageously the alignment sleeve is of glass, preferably silicaglass, such as fused silica glass, fused quartz and/or doped silica,and/or borosilicate glass, such as a borosilicate glass comprisingapproximately 96% silica and 4% boron trioxide e.g. such as the glasssold under the tradename Vycor™.

The doped glass may for example include fluoride doped silica, borondoped silica and/or germanium doped silica. By doping the glass, theglass may become more malleable and simpler to collapse.

It has been found to be advantageous that the said alignment sleeve isof silica with refractive index of up to 1.45 for light at 1 μm, such asfor light in the range 1-2 μm.

In an embodiment the alignment sleeve is of down doped silica, such assilica doped with fluorine and/or boron.

In an embodiment the alignment sleeve has a refractive index which isless than an effective refractive index of the cladding region.

Advantageously the alignment sleeve is substantially fully transparentfor the wavelengths of light transmitted or transmittable in the PCF,thereby the risk of excessive heating due to absorbing light is reducedor even avoided.

The alignment sleeve is preferably a capillary tube which has beenapplied to surround the PCF and collapsed by heat in at least thesupport section thereof such as a mid-section thereof to thereby beingarranged to support the PCF first fiber end section. This constructionis very simple and effective and thereby economically feasible.

By only collapsing a section of the alignment sleeve, the non-collapsedlength parts of the alignment sleeve may be shaped to have an outercross sectional diameter which fits to the cross sectional innerdiameter of the inner ferrule front section. Thereby, the inner diameterof the inner ferrule front section is correlated to the outer diameterof the non-collapsed alignment sleeve and PCFs with different outerdiameters can be mounted in the alignment sleeve by collapsing of asupport length section of the alignment sleeve.

Advantageously the alignment sleeve is hold within said inner ferrulefront section without any intermediate material between the alignmentsleeve and the inner ferrule front section. In an embodiment thealignment sleeve ismechanically hold and/or fused to said inner ferrulefront section.

Advantageously the inner ferrule front section surrounds and holds thealignment sleeve in a preselected axial position. The inner diameter ofthe inner ferrule front section is advantageously correlated to thealignment sleeve by being slightly larger than a maximal outer diameterof the alignment sleeve, such as slightly larger to allow the alignmentsleeve to be inserted into the bore of the hollow through hole of theinner ferrule front section e.g. from about 0.1 μm larger to about 2 mmlarger, such as from about 1 μm larger to about 1 mm larger, such asfrom about 0.1 mm larger to about 0.01 mm larger in diameter.

Advantageously the alignment sleeve surrounds the PCF proximally to thefirst fiber end. Preferably the first fiber end and an end of thealignment sleeve are aligned in a plane perpendicular to the PCF centeraxis.

The alignment sleeve may in principle have any length (determined inaxial direction), such as up to the length of the inner ferrule frontsection. In practice it is desired that the alignment sleeve isrelatively short, but sufficiently long to ensure a support of theelectromagnetic force. Advantageously, the alignment sleeve has a lengthin the axial direction which is least about 1 mm, such as from about 2mm to about 5 cm, such as from about 5 mm to about 2 cm. It has beenfound that for most PCF assemblies the optimal length of the alignmentsleeve is about 10 mm.

In an embodiment the alignment sleeve supports the first fiber endsection by having an inner diameter at least along a support sectionthereof which is adapted to the outer diameter of the first fiber endsection. In this embodiment it is desired that the inner diameter of thesupport section of the alignment sleeve is up to about 0.5 mm largerthan the inner diameter, such as up to about 0.1 mm, such as up to about0.01 mm. Where the alignment sleeve is crimped or collapsed onto thefirst fiber end section, the inner diameter of the alignment sleeve innon-collapsed or non-crimped state may be larger, such as up to about 5mm larger than the PCF diameter, such as up to about 3 mm larger, suchas up to about 1 mm larger than the PCF diameter.

In an embodiment the inner ferrule front section supports the firstfiber end section via the alignment sleeve and by mechanically holdingthe alignment sleeve in axial position, and an end of the alignmentsleeve and an end of the inner ferrule front section are aligned in aplane perpendicular to the ferrule structure center axis. Preferably thefirst fiber end, the end of the alignment sleeve and the end of theinner ferrule front section are aligned in a plane perpendicular to thePCF center axis.

In the assembling of the ferrule the alignment sleeve is moved in thez-direction to achieve the desired position of the fiber facet in thez-direction, e.g. relative to an outer ferrule reference point. Herebyyou ensure that the desired optical focal point in the ferrule isachieved. Subsequently, prior to or simultaneously the fiber is anchoredon the rear inner ferrule section.

In an embodiment the inner ferrule front section supports the firstfiber end section proximally to the first fiber end directly—with analignment sleeve—and preferably by mechanically holding the first fiberend section proximally to the first fiber end in axial position.Preferably an end of the inner ferrule front section and the first fiberend are aligned in a plane perpendicular to the PCF center axis.Advantageously is desired that the inner diameter of the inner ferrulefront section is up to about 0.5 mm larger than the inner diameter, suchas up to about 0.1 mm, such as up to about 0.01 mm.

In practical usage, some fraction of light at a light exiting end of thePCF may be guided in the cladding as cladding modes e.g. due toreflected and/or incident radiation further at the input end of the PCFsome fraction of light focused into and transmitted in a PCF fiber maynot be guided by the core but may be guided as cladding modes e.g. dueto mismatch of beam parameters into the fiber, focusing lensimperfections and dust/imperfections on optical surfaces and similar.These cladding modes may result in a very high temperature when used forhigh power transmission. And in particular the temperature at thetermination end of the PCF enclosed within the ferrule structure maybecome excessively high, which may damage a polymer coating of the PCFand thereby damage the whole termination and the fixation between theferrule structure and the PCF.

The component of the incident radiation that is not coupled into thecore will propagate within the cladding until it diverges to theprotective polymer coating where it is removed (“stripped”). If thesource of laser radiation input into the fiber is a high power laser,the intensity of radiation within these cladding modes is readilycapable of burning the protective polymer coating and destroying thefiber. An example is laser radiation onto industrial workpiece targets(particularly metallic targets) that is reflected with substantial powerback toward the fiber such that radiation couples into the claddingcircumference rather than into the core of the fiber. For this reason itis necessary to remove any cladding mode radiation at all fiberterminations before it can possibly diverge to and destroy theprotective polymer coating. “Mode stripping” is the name given to thenumerous techniques used to remove such cladding modes.

This problem has been solved by an embodiment of the invention whereinthe PCF is free of polymer from its anchoring length section to theinner ferrule rear section and to its first fiber end. In an embodimentthe ferrule structure comprises a hermetic solder element arranged tosurround the first fiber end section to form an annular hermetic sealbetween the first fiber end section and the inner ferrule rear section.The hermetic solder element is arranged closer to the front annularsection than the anchor length section of the inner ferrule rearsection. Further the anchoring length section of the inner ferrule rearsection is preferably not fully annular, thereby allowing heat todissipate from the PCF at the anchoring length section and preferably tothe hermetic solder element. The anchoring length section of the innerferrule rear section is preferably extending from about 20 degrees toabout 350 degrees, such as about 180 degrees (semi-annular) around thePCF.

The hermetic solder element additionally serves to protect the firstfiber end from dust and other undesired contamination. Where the PCFassembly comprises an end cap, the hermetic solder element ensures ahermetic seal of the first fiber end section from the first fiber endand to the position in z-direction of the hermetic solder element. Anypolymer coating of the PCF is advantageously stripped off the firstfiber end section from the first fiber end and to the position inz-direction of the hermetic solder element.

In an embodiment the first fiber end section is mounted in said ferrulestructure substantially without application of pressure to the PCF. Theapplication of pressure to the PCF may generate stress in the fiber,which for many applications is highly undesired because the beam qualitymay be of low quality or the stress may even damage the PCF or lower thelifetime of the PCF.

In an embodiment the first fiber end section is mounted in said ferrulestructure without any direct bonds to the fiber beyond one or more bondsto the inner ferrule rear section including the anchoring.

In an embodiment the PCF is free of polymer coating in the first fiberend section from the anchoring section of the inner ferrule rear sectionto the first fiber end.

Advantageously the inner ferrule front section is of an at least partlytransparent material at a wavelength between about 200 nm and about 4μm, e.g. fused or crystallized quartz, and the inner ferrule rearsection is of fused or crystallized quartz or of a metal or alloy. In anembodiment the inner ferrule front section is of substantially undopedsilica having a refractive index of up to 1.45 for light at 1 μm.

Generally it is desired that the inner ferrule rear section is of amaterial with a high conductivity and low thermal expansion in order toensure a high heat dissipation. It is therefore in particularly desiredthat the inner ferrule rear section is of a metal or metal alloy, suchas Colsibro® which is a high copper alloy with small additions of nickeland silicon, which serves to increase the strength, hardness and wearresistance of the material.

The outer ferrule arrangement primarily has the function of holding theinner ferrule arrangement section in position relative to each other,but advantageously the outer ferrule arrangement also aids in the heatdissipation from the ferrule structure.

Advantageously the outer ferrule arrangement is of metal, ceramic orglass e.g. silica.

In an embodiment the outer ferrule arrangement is at least partlytransparent to allow light to escape to an optional outer alignmentjacket where it may be absorbed. In particular light from cladding modesare allowed to escape via the outer ferrule arrangement, optionallycladding mode light stripped off as explained further below is allowedto escape via the outer ferrule arrangement.

In an embodiment the outer ferrule arrangement is fixed to each of theinner ferrule front section and the inner ferrule rear section of theinner ferrule arrangement to hold them in a fixed position relative toeach other, such that the first fiber end section of the PCF ispreferably supported to be substantially straight within the ferrulestructure. The outer ferrule arrangement is preferably fixed to theinner ferrule arrangement by glue, by solder and/or by being fused orlaser welded.

The PCF assembly may advantageously comprise an end cap arrangement e.g.for protecting the first fiber end against dust, moisture and similarcontaminations. In an embodiment the ferrule structure comprises an endcap arranged in front of the first fiber end. The end cap may be mountedwith a distance or without a distance to the inner ferrule front sectiondepending on the structure and the intended use of the PCF. The end capis preferably fixed directly to the inner ferrule front section or to anouter ferrule front section of the outer ferrule arrangement asdescribed further below.

In an embodiment the end cap is fixed to the outer ferrule front sectionof the outer ferrule arrangement. In this embodiment the outer ferrulearrangement preferably comprises the outer ferrule front section andouter ferrule rear section, wherein the outer ferrule rear section isfixed to both the inner ferrule rear section and the inner ferrule frontsection to hold these inner ferrule arrangement sections in relativepositions and the outer ferrule front section is fixed to the innerferrule front section and the end cap, thereby holding the end cap inposition relative to the first fiber end.

The end cap may be as prior art end caps e.g. with or without focusingelements. Advantageously the end cap is an anti-reflection coated silicaend cap. Where the end-cap is mounted with a distance to the first fiberend it is desired that the end cap comprises an anti-reflection coatingon both sides. This is in particular desired where the PCF is a hollowcore PCF.

In an embodiment the end cap is a lens. Preferably the lens comprises anantireflective coating on both of its sides i.e. its side facing thefirst fiber end and its opposite side.

The PCF may in principle be any kind of PCF, such as the PCFs discussedin the introduction above. In an embodiment the PCF is selected from ahollow core fiber, such as a bandgap fiber, a kagome fiber, or ananti-resonant-reflection (ARS) fiber, or a solid core fiber.

In an embodiment the PCF has a core diameter of less than 100 μm,preferably of about 50 μm or less, such as from about 5 μm to about 40μm.

In an embodiment the PCF at its first fiber end section comprises aterminal section of a hollow capillary fiber for protecting the end ofthe PCF from back reflections. This is achieved by positioning a pieceof hollow capillary fiber in front of the PCF, e.g. in the alignmentsleeve or in the inner ferrule front section.

In an embodiment the first fiber end (facet) has a metallic oranti-reflex coating.

In an embodiment the PCF is a hollow core fiber and the end cap is fixedto the outer ferrule front section of the outer ferrule arrangement toprovide an end cap space between the end cap and the inner ferrule frontsection. The cap space may in principle be very small determined in thez-direction, such a 1 mm and up to e.g. 5 cm. In practice the cap spaceis kept below about 1 cm for practical reasons.

In an embodiment the hollow core has a collapsed end part (such as up to2 mm in length, preferably up to 1 mm in length) and a metallic oranti-reflex coating on the fiber facet. In an embodiment the innerferrule arrangement comprises a passage into the end cap space forinjecting and/or withdrawing fluids, and in particular gas. The passageis preferably provided by at least one additional through hole in eachof the inner ferrule front section and the inner ferrule rear section.The additional through holes are preferably substantially parallel tothe axis of the ferrule structure. Preferably the additional throughhole comprises a valve arrangement at an exit from the inner ferrulerear section.

The passage into the end cap space for injecting and/or withdrawingfluids may advantageously be applied for injecting or flushing withsuitable gasses e.g. to ensure a moisture free hollow core. Suitablegasses include air, argon, nitrogen or mixtures comprising any of thementioned gasses. Optionally, the cap space for injecting and/orwithdrawing fluids are arranged for generating a hollow core pressure ofabout 1 mbar or less, such as to a pressure of about 0.1 mbar or less,such as to a pressure of about of 0.01 mbar or less at standardtemperature.

Optionally the cap space for injecting and/or withdrawing fluids arearranged for generate a hollow core pressure of up to 2 bars, such as upto about 1.5 bars at standard temperature.

In an embodiment the inner ferrule arrangement comprises at least onepassage into the gap between the inner ferrule front section and theinner ferrule rear section. The passage is advantageously a passagethrough the inner ferrule rear section, preferably in the form of athrough hole e.g. parallel to the center axis of the ferrule structure.The injecting and/or withdrawing of fluids, such as injecting and/orwithdrawing of the gasses mentioned above. In an embodiment the innerferrule arrangement comprises at least two passages into the gap betweenthe inner ferrule front section and the inner ferrule rear section forflushing the gap e.g. for drying out the gap and/or for heatdissipation.

In an embodiment the inner ferrule front section has a rear end where atleast an in radial direction outer part of the rear end is angledrelative to the center axis of the ferrule structure to out-couplelight, such as light propagating in the inner ferrule arrangement. In anembodiment the inner ferrule front section has a rear end where at leastan in radial direction outer semi annular or annular in radial directionouter part of the rear end is angled relative to the center axis of theferrule structure to out-couple light. The light may advantageously beout-coupled to pass through the outer ferrule arrangement.

In an embodiment the inner ferrule front section has a rear end where atleast an in radial direction inner part of the rear end is angledrelative to the center axis to form a funnel shape. Thereby the PCF maybe simpler to mount in the inner ferrule front section. The rear end ofthe inner ferrule front section may be angled in a part of its extensionaround the fiber or in its whole annular extension around the PCF.

The rear end of the inner ferrule front section may advantageously becoated with a reflective coating to back-reflect light, such as lightwhich is incidentally radiated from the first fiber end and/or lightthat is propagating in the inner ferrule arrangement.

In an embodiment the inner ferrule front section has a front end and thefront end is coated with a reflective coating to protect the ferrulestructure against incident and/or back reflected light.

In an embodiment the inner ferrule rear section has a front end, thefront end is angled relative to the center axis of the ferrule structureand/or the front end is coated with a reflective coating to protectagainst incident and/or back reflected light. The front end of the innerferrule rear section may be angled in a part of its extension around thefiber or in its whole annular extension around the PCF.

As explained above the ferrule structure of the PCF assembly of theinvention allows for several functions of stripping off undesiredcladding modes by reflecting and directly stripping off such claddingmodes. As explained in the following the invention allow for evenfurther functions of stripping off undesired light.

In an embodiment the first fiber end section has at least one modestripper length section. The mode stripper length section comprises amode stripping high index material and or/a scattering layer applied incontact with the optical fiber at the mode stripper length sectionand/or the fiber in the mode stripper length section has a roughness Ravalue of at least about 0.1 μm.

The high roughness Ra value may e.g. be provided by etching (e.g. laseretching or chemical etching) or mechanical grinding. The Ra-value may bemeasured in accordance with the ISO 4287, DIN 4762 and/or DIN 4768standards.

The mode stripper length section reduces or fully prevents forwardpropagating cladding light. The mode stripping high index material mayfor example be glue with silica particles e.g. doped to increase therefractive index and/or chemical glass.

In an embodiment the mode stripper length section of the PCF ispositioned between the inner ferrule front section and the inner ferrulerear section.

In an embodiment at least one of the inner ferrule front section and theinner ferrule rear section has a carving exposing the PCF the modestripper length section of the PCF.

Advantageously the carving preferably partly surrounds the fiberpreferably such that it extends at least about 20 degrees and e.g. up toabout 350 degrees, preferably from about 20 degrees to about 90 degreesin the direction around the PCF.

In an embodiment the ferrule structure comprises a mode stripper coatingarranged in direct contact with an outer surface of the inner ferrulefront section, the mode stripper coating is preferably contained betweenthe inner ferrule front section and the outer ferrule arrangement.

In an embodiment the ferrule structure comprises an outer alignmentjacket surrounding the outer ferrule arrangement, the outer alignmentjacket preferably comprises means for alignment, preferably foralignment in axial direction (z-direction) and/or for rotationalalignment.

The main purpose of the outer alignment jacket is to ensure a rigid andmechanical strength of the ferrule structure and simpler alignment ofthe PCF with respect to an emitted beam or in-coupled beam of light.Further for axial and rotational alignment the outer alignment jacket isvery beneficial.

In an embodiment the means for alignment comprises means for alignmentin the axial direction (z-direction), for alignment in the radialdirections (x,y-directions) and/or for rotational alignment.

Advantageously the means for alignment comprises one or more flanges,one or more protrusions, one or more depressions and/or one or moremarkings.

Preferably the means for alignment comprises a flange for mounting withpositioning control.

In an embodiment the means for alignment comprises a marker forrotational fiber orientation e.g. for PM orientation.

Generally it is desirable to be able to rotate the fiber in the ferruleso that the PM axis is aligned with respect to the means for alignmenton the outside of the ferrule). This may e.g. be provided by having acorresponding marking on the inner ferrule rear section immediatelyadjacent to the anchor length section.

In an embodiment the ferrule structure is configured for cooling by acooling fluid the outer ferrule arrangement and/or the outer alignmentjacket comprises passages with at least one inlet and at least one exitfor the cooling fluid, such as water. The passages in the outer ferrulearrangement and/or the outer alignment jacket are advantageouslyarranged to helically surround the underlying element(s) to provide goodheat dissipation. By ensuring that the outer ferrule arrangement istransparent to light stripped off and/or out-coupled via the innerferrule arrangement, this light may escape via the outer ferrulearrangement and be absorbed by outer alignment jacket. Thereby the outeralignment jacket will be heated, but due to the cooling of the outeralignment jacket by a cooling fluid, the temperature may be held at anacceptable level even when the PCF is transporting light at high power.

In an embodiment the assembly further comprises one or more sensors,such as one or more optical sensors and/or electrical sensors and/orchemical sensors for monitoring temperature, for monitoring connectorperformance and/or for monitoring fiber damage. The sensors areadvantageously arranged in the ferrule structure, such as the outerferrule arrangement and/or the outer alignment jacket. Advantageouslythe sensors are fiber sensors, but in principle also non-fiber sensorsare applicable.

In an embodiment the assembly comprises a second ferrule structureconnected to a second end section of the PCF comprising a second fiberend. The second ferrule structure preferably is as the ferrule structureas described above and is mounted to the second end section of the PCFin a corresponding way as described above. In an embodiment the secondend of the PCF is a fiber spliced end and/or it is coupled comprisingfree space coupling.

The invention also comprises a laser system comprising a PCF assembly asdescribed above.

The laser system advantageously comprises a laser light source and thePCF assembly is optically connected to the laser light source forreceiving light from the laser light source. The laser light source mayin an embodiment be arranged for directly feed the light to the PCF e.g.by being fused to the PCF. In an embodiment the laser light source isarranged for feeding the light to the PCF via one or more opticalelements and/or via free space.

Preferably the PCF assembly is adapted for delivering the light to alight employing station of an apparatus. Preferably the first fiber endwith the ferrule structure is adapted for being connected to the userapparatus.

The apparatus is for example a microscope, surgical apparatus, measuringapparatus (metrology), materials processing apparatus, an illuminationapparatus, any combinations thereof and/or an apparatus as describedfurther below.

The laser light source may in principle be any kind of laser lightsource, such as a CW laser light source or a pulsed laser light source.Such laser light sources are well known in the art and will not bedescribed in further details herein.

In an embodiment the laser light source is configured for generatinglaser light pulses, preferably the laser light source is a femtosecondlaser source or a picosecond laser light source or a nanosecond laserlight source.

In an embodiment the laser light source has a pump duration of fromabout 30 fs to about 30 ps, such as from about 100 fs to about 10 ps.

In an embodiment the laser light source has a peak power determined atthe exit of the laser light source which is at least about 5 kW, such asat least about 10 kW, such as at least about 30 kW, such as at leastabout 50 kW.

The laser light source is advantageously a mode-locked laser lightsource. In an embodiment laser light source is an actively mode lockedlaser. In an embodiment the laser light source is a passively modelocked laser. The mode locked laser preferably comprises one or moreamplifiers.

In an embodiment the laser system is configured for supercontinuumgeneration and the laser light source is a mode-locked pump pulse lightsource arranged for feeding the PCF to generate supercontinuum. The PCFis preferably a solid core PCF suitable for supercontinuum generationsuch as a PCF described in WO15144181, WO15003715, WO15003714,US2015192732 and/or US2011013652.

In an embodiment where the PCF is a solid core PCF, it is desired thatthe PCF is a microstructured solid core PCF comprising a plurality ofnon-solid and/or solid cladding inclusions. The solid core PCF ispreferably configured for guiding light—preferably single modelight—comprising at least one wavelength in the range from about 200 nmto about 4.5 μm, preferably at least one wavelength in the range from1000 nm to about 1100 nm.

In an embodiment the PCF is a hollow core PCF, preferably the hollowcore PCF is configured for guiding light—preferably single modelight—comprising at least one wavelength in the range from about 200 nmto about 4.5 μm, preferably at least one wavelength in the range from1000 nm to about 1100 nm.

In an embodiment the PCF is configured for guiding a continuum of lightwavelengths, preferably spanning over at least about 0.1 μm, such as atleast about 0.3 μm, such as at least about 0.5 μm.

In an embodiment the hollow core PCF comprises an outer cladding regionand a number N of hollow tubes surrounded by the outer cladding region,wherein each of the hollow tubes is fused to the outer cladding to forma ring defining an inner cladding region and the hollow core regionsurrounded by the inner cladding region, preferably N is from 6 to 12,more preferably N is 7. Advantageously the hollow tubes are not touchingeach other, preferably the hollow tubes are arranged with asubstantially equal distance to adjacent hollow tubes.

The hollow core PCF is advantageously as described in the co-pendingapplication PA 2015 70877 DK by the same applicant and with the title“HOLLOW CORE OPTICAL FIBER AND A LASER SYSTEM”.

The PCF of the laser system advantageously has a core region diameter offrom about 3 μm to about 100 μm, such as from about 10 μm to about 50μm, such as from about 10 μm to about 30 μm.

The invention also comprises a set of correlated ferrule elementssuitable for a PCF assembly as described above.

The set of correlated ferrule element comprises the required elementsfor providing a ferrule structure for a PCF to generate a PCF assemblyas described above.

The set of correlated ferrule elements comprises

-   -   an inner ferrule front section and an inner ferrule rear section        for forming an inner ferrule arrangement; and    -   an outer ferrule arrangement,        wherein each of the inner ferrule front section, inner ferrule        rear section and outer ferrule arrangement has a length and a        center axis and comprises a main hollow through hole parallel        with or coincident to the respective center axes, the set of        correlated ferrule elements preferably further comprises an        alignment sleeve having a length and a center axis and comprises        a main hollow through hole parallel with or coincident to the        center axis.

Advantageously the elements are correlated such that the alignmentsleeve can be positioned in the main hollow through hole of the innerferrule front section and the inner ferrule front section and the innerferrule rear section can be mounted in the main hollow through hole ofthe outer ferrule arrangement for forming the inner ferrule arrangement.Thereby the set of correlated ferrule elements may be assembled with aPCF to a PCF assembly.

The alignment sleeve is advantageously a capillary tube wherein the mainhollow through hole of the alignment sleeve is has an inner diameterwhich is about 2 mm or less, such as about 1 mm or less such as about0.5 mm or less, the alignment sleeve is preferably collapsible in atleast a part of its length as described above.

The alignment sleeve may advantageously be as described above and of amaterial as described above.

In an embodiment the alignment sleeve has a length in axial directionwhich is preferably at least about 1 mm, such as from about 2 mm toabout 5 cm.

The inner ferrule front section and the inner ferrule rear section aswell as the outer ferrule arrangement are advantageously as describedabove and of materials as described above.

In an embodiment the set further comprises an end cap which isconfigured for being arranged in front of the inner ferrule frontsection by being mounted to the inner ferrule front section or by beingmounted to an outer ferrule front section of the outer ferrulearrangement. The end cap is preferably an anti-reflection coated silicaend cap e.g. as described above.

In an embodiment each of the inner ferrule front section and the innerferrule rear section comprises one or more additional through holes forproviding a fluid passage, the additional through holes are preferablysubstantially parallel to the axis of the respective inner ferrulesections and optionally the additional through hole at an exit from theinner ferrule rear section comprises a valve arrangement e.g. asdescribed above.

In an embodiment the inner ferrule front section has a rear end, atleast an in radial direction outer part of the rear end is angledrelative to the center axis of the inner ferrule front section and/orthe rear end is coated with a reflective coating in order to out-couplelight preferably as described above.

In an embodiment the inner ferrule front section has a rear end, atleast an in radial direction inner part of the rear end is angledrelative to the center axis to form a funnel shape to thereby make itsimpler to insert the PCF into the inner ferrule front section e.g. asdescribed above.

Advantageously the inner ferrule front section has a front end, thefront end is coated with a reflective coating.

In an embodiment the inner ferrule rear section has a front end and thefront end is angled relative to the center axis of the inner ferrulerear section and/or the front end is coated with a reflective coating,preferably as described above.

In an embodiment at least one of the inner ferrule front section and theinner ferrule rear section has a carving into its main hollow throughhole, the carving preferably has an extension in an annular directionwhich extends at least about 20 degrees such as up to about 350 degrees,such as up to about 90 degrees.

In an embodiment the set further comprises an outer alignment jacketcorrelated to the outer ferrule arrangement such that it can be arrangedto surround the outer ferrule arrangement e.g. as described above. Theouter alignment jacket preferably comprises means for alignment e.g. asdescribed above.

Advantageously the outer ferrule arrangement and/or the outer alignmentjacket comprises passages with at least one inlet and at least one exitfor a cooling fluid e.g. as described above.

The invention also comprises an apparatus comprising a laser system asdescribed above, wherein the PCF assembly is configured for deliveringlight to a light receiving station of the apparatus.

The apparatus may in principle be any kind of apparatus, which useslaser light in its operation. In an embodiment the apparatus is anillumination apparatus configured for illuminating a target, theillumination apparatus is preferably selected from a microscope, aspectroscope or an endoscope.

In an embodiment the illumination source is adapted for fluorescenceImaging; Fluorescence Lifetime Imaging (FLIM); Total Internal ReflectionFluorescence (TIRF) Microscopy; fluorescence resonance energy transfer(FRET); pulse interleave excitation foster resonance energy transfer(PIE-FRET); broadband Spectroscopy; nanophotonics; flow cytometry;industrial inspection, such as metrology; ringdown spectroscopy, such asgas sensing; analytical spectroscopy, such as hyperspectralspectroscopy, crop analysis e.g. of fruits and time of flightspectroscopy (TCSPC); single Molecule Imaging and/or combinationsthereof.

In an embodiment the apparatus is a microprocessing apparat, preferablyfor material processing, such as drilling, marking, cutting and/orscribing.

In an embodiment the apparatus is a surgery apparat, such as anapparatus for eye surgery (opthamology).

All features of the inventions and embodiments of the invention asdescribed above including ranges and preferred ranges can be combined invarious ways within the scope of the invention, unless there arespecific reasons not to combine such features.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and/or additional objects, features and advantages of thepresent invention, will be further elucidated by the followingillustrative and non-limiting detailed description of embodiments of thepresent invention, with reference to the appended drawings.

FIG. 1 is a cross sectional view of a first embodiment of a PCF assemblyaccording to the invention, where in radial direction inner part of saidrear end is angled relative to the center axis to form a funnel shape.

FIG. 2 is a cross sectional view of a second embodiment of a PCFassembly according to the invention, where the inner ferrule rearsection has a passage into the gap between the inner ferrule frontsection and the inner ferrule rear section.

FIG. 3 is a cross sectional view of a third embodiment of a PCF assemblyaccording to the invention, where the PCF at its first fiber end sectionhas a mode stripper section and the inner ferrule front section has acarving exposing the PCF said mode stripper length section.

FIG. 4 is a cross sectional view of a fourth embodiment of a PCFassembly according to the invention, where the PCF at its first fiberend section has a mode stripper section and the inner ferrule rearsection has a carving exposing the PCF said mode stripper lengthsection.

FIG. 5 is a cross sectional view of a fifth embodiment of a PCF assemblyaccording to the invention, where a part of the rear end of the innerferrule front section is angled relative to the center axis of theferrule structure to out-couple light propagating in the inner ferrulearrangement.

FIG. 6 is a cross view of a sixth embodiment of a PCF assembly accordingto the invention, where the rear end of the inner ferrule rear sectionin its entire annular extension is angled relative to the center axis ofthe ferrule structure to out-couple light propagating in the innerferrule arrangement.

FIG. 7 is a cross sectional view of a seventh embodiment of a PCFassembly according to the invention, where the PCF at its first fiberend section has a mode stripper section in the gap between the innerferrule front section and the inner ferrule rear section.

FIG. 8 is a cross sectional view of an eighth embodiment of a PCFassembly according to the invention, where the PCF assembly comprises anend cap mounted to the inner ferrule front section.

FIG. 9a is a cross sectional view of a ninth embodiment of a PCFassembly according to the invention, where the PCF assembly comprises anend cap mounted to the outer ferrule front section.

FIG. 9b is a cross sectional view of a PCF assembly which is a variationof the ninth embodiment shown in FIG. 9 a.

FIG. 10 is a cross sectional view of a tenth embodiment of a PCFassembly according to the invention, where the PCF assembly comprises anouter alignment jacket.

FIG. 11 is a cross sectional view of an eleventh embodiment of a PCFassembly according to the invention, where the PCF assembly comprises analignment sleeve.

FIG. 12a is a cross sectional view of a twelfth embodiment of a PCFassembly according to the invention, where the PCF assembly comprises analignment sleeve and the PCF comprises a section of hollow corecapillary.

FIG. 12b is a cross sectional view is an enlarged cross sectional viewof the alignment sleeve and supported first fiber end section of the PCFassembly of FIG. 12 a.

FIG. 13a is a cross sectional view of a thirteenth embodiment of a PCFassembly according to the invention, where the PCF assembly comprises analignment sleeve with a collapsed supporting section.

FIG. 13b is a cross sectional view is an enlarged cross sectional viewof the alignment sleeve and supported first fiber end section of the PCFassembly of FIG. 13 a.

FIGS. 14a, 14b, 14c and 14d show schematic cross sectional views of PCFfiber end sections.

FIG. 15 is a schematic drawing of a laser system of an embodiment of theinvention and a user apparatus.

FIG. 16 is a schematic drawing of an apparatus of an embodiment of theinvention and a user apparatus.

FIGS. 17a and 17b are schematic drawings of an apparatus of anembodiment of the invention and a user apparatus where the laser systemis a supercontinuum laser system.

The figures are schematic and may be simplified for clarity. Throughout,the same reference numerals are used for identical or correspondingparts.

The PCF assembly of FIG. 1 comprises a photonic crystal fiber (PCF)assembly PCF with a first fiber end section 1 with a first fiber end 1 aand a ferrule structure. The ferrule structure comprises an innerferrule arrangement comprising an inner ferrule front section 2proximally to the first fiber end and 1 a an inner ferrule rear section3 distally to first fiber end 1 a. The inner ferrule front section 2 andthe inner ferrule rear section 3 respectively comprises hollow throughholes 2 a, 3 a coincident with center axes of the inner ferrule frontsection 2 and inner ferrule rear section. The PCF first fiber endsection is mounted in said hollow through holes 2 a, 3 a, such that theinner ferrule arrangement surrounds the first fiber end section.

The inner ferrule rear section 3 is anchored in an anchor length section3 b to said first fiber end section 1, and from a point 1 c of the PCFand to the first fiber end the PCF is free of polymer coating. The innerferrule front section 2 supports the first fiber end section 1proximally to the first fiber end 1 a by mechanically holding said firstfiber end section proximally to the first fiber end in axial position.As it can be seen the front of the inner ferrule front section and thefirst fiber end are aligned in a plane perpendicular to the PCF centeraxis.

The ferrule structure further comprises an outer ferrule arrangement 5arranged to surround the inner ferrule arrangement. The outer ferrulearrangement 5 is fixed to each of the inner ferrule front section 2 andthe inner ferrule rear section 3 to hold them in a fixed positionrelative to each other and to form a gap 4 there between. The outerferrule arrangement 5 is fixed by solder 6 at each of its ends to therespective sections 2, 3 of the inner ferrule arrangement.

The PCF first fiber end section is advantageously held relativelystraight in the gap 4 between the inner ferrule front section 2 and theinner ferrule rear section 3. In an alternative not shown embodiment thePCP has a surplus length between the inner ferrule front section 2 andthe inner ferrule rear section 3.

The in radial direction inner part 2 c of the rear end of the innerferrule front section 2 is angled relative to the center axis to form afunnel shape, which makes it simpler to feed the PCF 1 into the innerferrule front section during assembling. The in radial direction outerpart 2 d of the rear end of the inner ferrule front section 2 isadvantageously coated with angled reflective coating to back-reflectlight propagating in the inner ferrule arrangement.

Further the front end 2 e of the inner ferrule front section 2 ispreferably coated with a reflective coating to protect the ferrulestructure against incident and/or back-reflected light and the front end3 e of the inner ferrule rear section is preferably coated with areflective coating to protect against incident and/or back-reflectedlight.

The PCF assembly of FIG. 2 comprises a photonic crystal fiber (PCF)assembly PCF with a first fiber end section 11 with a first fiber end 11a and a ferrule structure. The ferrule structure comprises an innerferrule arrangement comprising an inner ferrule front section 12proximally to the first fiber end and 11 a an inner ferrule rear section13 distally to first fiber end 11 a. The inner ferrule rear section 13is anchored in an anchor length section 13 b to the first fiber endsection 11, and from a point 11 c of the PCF and to the first fiber endthe PCF is free of polymer coating. The ferrule structure furthercomprises an outer ferrule arrangement 15 arranged fixed to each of theinner ferrule front section 12 and the inner ferrule rear section 13 tohold them in a fixed position relative to each other. The front end 3 eof the inner ferrule rear section is preferably coated with a reflectivecoating to protect against incident and/or back-reflected light.

The front end 13 e of the inner ferrule rear section 13 is angledrelative to the center axis of the ferrule structure to form an outwardsfacing facet for out-coupling ferrule light. The outer ferrulearrangement 15 is preferably substantially transparent to theout-coupled light. The inner ferrule rear section 13 further comprises apassage 13 f into the gap 14 between the inner ferrule front section 12and the inner ferrule rear section 15. The passage may be used to fillin or withdraw fluids from the gap 14 as described above.

The PCF assembly of FIG. 3 comprises a photonic crystal fiber (PCF)assembly PCF with a first fiber end section 21 with a first fiber end 21a and a ferrule structure. The ferrule structure comprises an innerferrule arrangement comprising an inner ferrule front section 22proximally to the first fiber end 21 a and an inner ferrule rear section23 distally to first fiber end 21 a and surrounding the first fiber endsection 21.

The inner ferrule rear section 23 is anchored in an anchor lengthsection 23 b to said first fiber end section 21, and from a point 21 cof the PCF and to the first fiber end the PCF is free of polymercoating. The inner ferrule front section 22 supports the first fiber endsection 21 proximally to the first fiber end 21 a by mechanicallyholding said first fiber end section proximally to the first fiber endin axial position.

The ferrule structure further comprises an outer ferrule arrangement 25fixed to each of the inner ferrule front section 22 and the innerferrule rear section 23 to hold them in a fixed position relative toeach other and to form a gap 24 there between. The outer ferrulearrangement 25 is fixed by solder 26 at each of its ends to therespective sections 22, 23 of the inner ferrule arrangement.

The front end 23 e of the inner ferrule rear section 23 is angledrelative to the center axis of the ferrule structure to form an outwardsfacing facet for out-coupling light propagating in the inner ferrulearrangement and reduce back reflections. The inner ferrule rear section23 further comprises a passage 23 f into the gap 24 between the innerferrule front section 22 and the inner ferrule rear section 25. Thepassage may be used to fill in or withdraw fluids from the gap 24 asdescribed above. The PCF at its first fiber end section 21 has a modestripper section 27 and the inner ferrule front section 22 has a carving27 a exposing the PCF mode stripper length section 27.

A sensor 28 is mounted to the outer ferrule arrangement 25 above thecarving 27 a exposing the PCF mode stripper length section formonitoring the out-coupling efficiency of the mode stripper 27.

The PCF assembly of FIG. 4 comprises a photonic crystal fiber (PCF)assembly PCF with a first fiber end section 31 assembled with a ferrulestructure comprising an inner ferrule arrangement comprising an innerferrule front section 32 proximally to the first fiber end and an innerferrule rear section 33 distally to first fiber end and surrounding thefirst fiber end section 31.

The inner ferrule rear section 33 is anchored to the first fiber endsection 31, and from a point 31 c of the PCF and to the first fiber endthe PCF is free of polymer coating. The inner ferrule front section 32supports the first fiber end section 31 proximally to the first fiberend 31 a by mechanically holding said first fiber end section proximallyto the first fiber end in axial position.

The ferrule structure further comprises an outer ferrule arrangement 35fixed to each of the inner ferrule front section 32 and the innerferrule rear section 33 to hold them in a fixed position relative toeach other and to form a gap 34 there between. The outer ferrulearrangement 35 is fixed by solder 36 at each of its ends to therespective sections 32, 33 of the inner ferrule arrangement.

The PCF at its first fiber end section 31 has a mode stripper section 37and the inner ferrule rear section 33 has a carving 37 a exposing thePCF mode stripper length section 37.

A sensor 38 is mounted to the outer ferrule arrangement 35 above thecarving 37 a exposing the PCF mode stripper length section formonitoring the out-coupling efficiency of the mode stripper 37.

The PCF assembly of FIG. 5 comprises a photonic crystal fiber (PCF)assembly PCF with a first fiber end section 41 assembled with a ferrulestructure comprising an inner ferrule arrangement comprising an innerferrule front section 42 proximally to the first fiber end and an innerferrule rear section 43 distally to first fiber end and surrounding thefirst fiber end section 41.

The inner ferrule rear section 43 is anchored to the first fiber endsection 41, and from a point 41 c of the PCF and to the first fiber endthe PCF is free of polymer coating. The inner ferrule front section 42supports the first fiber end section 41 proximally to the first fiberend 41 a by mechanically holding said first fiber end section proximallyto the first fiber end in axial position.

The ferrule structure further comprises an outer ferrule arrangement 45fixed to each of the inner ferrule front section 42 and the innerferrule rear section 43 to hold them in a fixed position relative toeach other and to form a gap 44 there between. The outer ferrulearrangement 45 is fixed by solder 46 at each of its ends to therespective sections 42, 43 of the inner ferrule arrangement.

The PCF at its first fiber end section 41 has a mode stripper section 47and the inner ferrule rear section 43 has a carving 47 a exposing thePCF mode stripper length section 47.

The front end 43 e of the inner ferrule rear section 43 is angledrelative to the center axis of the ferrule structure to form an outwardsfacing facet for out-coupling light propagating in the inner ferrulearrangement and reduce back-reflection of light. A part 42 b—for examplea semi-annular part of the rear end of the inner ferrule frontsection—is angled relative to the center axis of the ferrule structureto out-couple light propagating in the inner ferrule arrangement andreduce back-reflections of light. The remaining part 42 c is not angledbut has a facet which is substantially perpendicular to the center axis.The not angled part 42 c of the rear end of the inner ferrule frontsection is advantageously coated with a reflective coating for reducingback-reflection of light.

Two sensors 48 a, 48 b are mounted to the outer ferrule arrangement 45above respectively the rear end of the inner ferrule front section andthe carving 47 a exposing the PCF mode stripper length section formonitoring the out-coupling efficiency of respectively the angled part42 b of the inner ferrule front section and the mode stripper 47.

The PCF assembly of FIG. 6 is a variation of the PCF assembly of FIG. 5with the modification that the entire rear end 42 b of the rear end ofthe inner ferrule front section is angled relative to the center axis ofthe ferrule structure to out-couple light propagating in the innerferrule arrangement and reduce back-reflections of light.

The PCF assembly of FIG. 7 comprises a photonic crystal fiber (PCF)assembly PCF with a first fiber end section 51 assembled with a ferrulestructure comprising an inner ferrule arrangement comprising an innerferrule front section 52 proximally to the first fiber end and an innerferrule rear section 53 distally to first fiber end and surrounding thefirst fiber end section 51.

The inner ferrule rear section 53 is anchored to the first fiber endsection 51, and from a point 51 c of the PCF and to the first fiber end51 a, the PCF is free of polymer coating. The inner ferrule frontsection 52 supports the first fiber end section 51 proximally to thefirst fiber end 51 a by mechanically holding said first fiber endsection proximally to the first fiber end in axial position.

The ferrule structure further comprises an outer ferrule arrangement 55fixed to each of the inner ferrule front section 52 and the innerferrule rear section 53 to hold them in a fixed position relative toeach other and to form a gap 54 there between. The outer ferrulearrangement 55 is fixed by solder 56 at each of its ends to therespective sections 52, 53 of the inner ferrule arrangement.

The PCF at its first fiber end section 51 has a mode stripper section 57positioned between the inner ferrule front section 52 and the innerferrule rear section 53.

The PCF assembly of FIG. 8 comprises a photonic crystal fiber (PCF)assembly PCF with a first fiber end section 61 assembled with a ferrulestructure comprising an inner ferrule arrangement comprising an innerferrule front section 62 proximally to the first fiber end and an innerferrule rear section 63 distally to first fiber end and surrounding thefirst fiber end section 61.

The inner ferrule rear section 63 is anchored to the first fiber endsection 61 in an anchor length section 63 b, and from a point 61 c ofthe PCF and to the first fiber end 61 a, the PCF is free of polymercoating.

The ferrule structure comprises a hermetic solder element 66 a arrangedto surround the first fiber end section 61 to form an annular hermeticseal 66 a between the first fiber end section 61 and the inner ferrulerear section 63, the hermetic solder element 66 a is arranged closer tothe inner ferrule front section 62 than the anchor length section 63 bof the inner ferrule rear section. As seen in the drawing the anchoringlength section 63 b of the inner ferrule rear section 63 is not fullyannular, the anchoring length section 63 of the inner ferrule rearsection is preferably extending from about 20 degrees to about 350degrees, such as about 180 degrees around the PCF.

The ferrule structure further comprises an outer ferrule arrangement 65fixed to each of the inner ferrule front section 62 and the innerferrule rear section 63 to hold them in a fixed position relative toeach other and to form a gap 64 there between. The outer ferrulearrangement 65 is fixed by solder 66 at each of its ends to therespective sections 62, 63 of the inner ferrule arrangement.

The ferrule structure further comprises an end cap 67 arranged in frontof the first fiber end 61 c and preferably in direct contact with thefirst fiber end 61 c. The end cap 67 is fixed directly to the innerferrule front section. As explained above this embodiment isparticularly beneficial where the PCF is a solid core PCF.

The PCF assembly of FIG. 9a comprises a photonic crystal fiber (PCF)assembly PCF with a first fiber end section 71 assembled with a ferrulestructure comprising an inner ferrule arrangement comprising an innerferrule front section 72 proximally to the first fiber end and an innerferrule rear section 73 distally to first fiber end and surrounding thefirst fiber end section 71.

The inner ferrule rear section 73 is anchored to the first fiber endsection 71 in an anchor length section 73 b, and from a point 71 c ofthe PCF and to the first fiber end 71 a, the PCF is free of polymercoating.

The ferrule structure comprises a hermetic solder element 76 a arrangedto surround the first fiber end section 71 to form an annular hermeticseal 76 a between the first fiber end section 71 and the inner ferrulerear section 73, the hermetic solder element 76 a is arranged closer tothe front annular section 72 than the anchor length section 73 b of theinner ferrule rear section.

The hermetic solder element ensures a hermetic seal of the first fiberend section 71 from the first fiber end 71 c and to the position inz-direction of the hermetic solder element 76 a.

The ferrule structure further comprises an outer ferrule arrangement 75,75 a comprising an outer ferrule front section 75 a and an outer ferrulerear section 75. The outer ferrule rear section 75 is fixed to each ofthe inner ferrule front section 72 and the inner ferrule rear section 73to hold them in a fixed position relative to each other and to form agap 74 there between. The outer ferrule rear section 75 is fixed bysolder 76 at each of its ends to the respective sections 72, 73 of theinner ferrule arrangement and the outer ferrule front section 75 a isfixed by solder 76 to the inner ferrule front section 72.

The ferrule structure further comprises an end cap 77 arranged in frontof the first fiber end 71 c. The end cap 77 is mounted with a distanceto the inner ferrule front section 72, thereby forming an end cap space78 between the end cap 77 and the inner ferrule front section 72. Theend cap is fixed to the outer ferrule front section 75 a of the outerferrule arrangement. Thereby the outer ferrule front section 75 a of theouter ferrule arrangement holds the end cap 77 in a desired positionrelative to the inner ferrule front section 72 and the first fiber endsection 71 c. As explained above this embodiment is particularlybeneficial where the PCF is a hollow core PCF.

The inner ferrule arrangement 72, 73 comprises a passage into the endcap space 78 for injecting and/or withdrawing fluids. The passage isprovided by additional through holes 72 f, 73 f in each of the innerferrule front section 72 and the inner ferrule rear section 73.Advantageously a not shown valve arrangement is arranged to ensure adesired open/closing function into the through holes 72 f, 73 f and theend cap space 78.

The PCF assembly shown in FIG. 9b , differs from the PCF assembly ofFIG. 9a in that the end cap is a lens 77 a, preferably the lens 77 acomprises an antireflective coating on both of its sides.

The PCF assembly of FIG. 10 comprises a photonic crystal fiber (PCF)assembly PCF with a first fiber end section 81 assembled with a ferrulestructure comprising an inner ferrule arrangement comprising an innerferrule front section 82 proximally to the first fiber end and an innerferrule rear section 83 distally to first fiber end and surrounding thefirst fiber end section 81.

The inner ferrule rear section 83 is anchored to the first fiber endsection 81 in an anchor length section 83 b, and from a point 81 c ofthe PCF and to the first fiber end 81 a, the PCF is free of polymercoating.

The ferrule structure comprises a hermetic solder element 86 a arrangedto surround the first fiber end section 81 to form an annular hermeticseal 86 a between the first fiber end section 81 and the inner ferrulerear section 83.

The hermetic solder element 86 a ensures a hermetic seal of the firstfiber end section 81 from the first fiber end 81 c and to the positionin z-direction of the hermetic solder element 86 a.

The outer ferrule rear section 85 is fixed to each of the inner ferrulefront section 82 and the inner ferrule rear section 83 to hold them in afixed position relative to each other and to form a gap there between.The outer ferrule rear section 85 is fixed by solder 86 at each of itsends to the respective sections 82, 83 of the inner ferrule arrangementand the outer ferrule front section 85 a is fixed by solder 86 to theinner ferrule front section 82.

The ferrule structure further comprises an end cap 87 arranged in frontof the first fiber end 81 c. The end cap 87 is mounted with a distanceto the inner ferrule front section 82, thereby forming an end cap space88 between the end cap 87 and the inner ferrule front section 82. Theend cap is fixed to the outer ferrule front section 85 a of the outerferrule arrangement.

The ferrule structure comprises an outer alignment jacket 89 surroundingthe outer ferrule arrangement 85, 85 a, the outer alignment jacketpreferably comprises means 89 a, 89 b for alignment, including in theshown embodiment a flange 89 a for alignment and a protrusion 89 b forrotational alignment e.g. for rotational fiber orientation.

The PCF assembly of FIG. 11 comprises a photonic crystal fiber (PCF)assembly PCF with a first fiber end section 91 assembled with a ferrulestructure comprising an inner ferrule arrangement comprising an innerferrule front section 92 proximally to the first fiber end and an innerferrule rear section 93 distally to first fiber end and surrounding thefirst fiber end section 91.

The assembly further comprises an alignment sleeve 90 arranged betweenthe inner ferrule front section 92 and the first fiber 91 end section tofully surround the first fiber end section 91, such that the innerferrule front section 92 supports the first fiber end section 91proximally to the first fiber end 91 a via the alignment sleeve 90. Inthe shown embodiment of FIG. 11 the alignment sleeve 90 is a shortsection of a capillary tube.

The front end of the alignment sleeve 90, the front end of the innerferrule front section 92 are and the first fiber end 91 a are aligned ina plane perpendicular to the ferrule structure center axis.

The inner ferrule rear section 93 is anchored to the first fiber endsection 91 in an anchor length section 93 b, and from a point 91 c ofthe PCF and to the first fiber end 91 a, the PCF is free of polymercoating.

The ferrule structure comprises a hermetic solder element 96 a arrangedto surround the first fiber end section 91 to form an annular hermeticseal 96 a between the first fiber end section 91 and the inner ferrulerear section 93.

The outer ferrule rear section 95 is fixed to each of the inner ferrulefront section 92 and the inner ferrule rear section 93 to hold them in afixed position relative to each other and to form a gap there between.The outer ferrule rear section 95 is fixed by solder 96 at each of itsends to the respective sections 92, 93 of the inner ferrule arrangementand the outer ferrule front section 95 a is fixed by solder 96 to theinner ferrule front section 92.

The ferrule structure further comprises an end cap 97 arranged in frontof the first fiber end 91 c. The end cap 97 is mounted with a distanceto the inner ferrule front section 92. The end cap is fixed to the outerferrule front section 85 a of the outer ferrule arrangement.

The ferrule structure comprises an outer alignment jacket 99 surroundingthe outer ferrule arrangement 95, 95 a, the outer alignment jacketpreferably comprises means 99 a, 99 b for alignment.

A sensor 98 in mounted to the outer alignment jacket 99 for monitoringthe connector performance and/or for monitoring fiber damage.

The PCF assembly of FIG. 12a and FIG. 12b comprises a photonic crystalfiber (PCF) assembly PCF with a first fiber end section 101 assembledwith a ferrule structure comprising an inner ferrule arrangementcomprising an inner ferrule front section 102 proximally to the firstfiber end and an inner ferrule rear section 103 distally to first fiberend and surrounding the first fiber end section 101.

The assembly further comprises an alignment sleeve 100 arranged betweenthe inner ferrule front section 102 and the first fiber 101 end sectionto fully surround the first fiber end section 101, such that the innerferrule front section 102 supports the first fiber end section 101proximally to the first fiber end 101 a via the alignment sleeve 100. Inthe shown embodiment of FIG. 11 the alignment sleeve 100 is a shortsection of a capillary tube. The alignment sleeve 100 and the supportedfirst fiber end section 101 are enlarged in FIG. 12a and it can be seenthat the PCF comprises a short section of another fiber 101 b, which isadvantageously a section of hollow core capillary 101 b.

The front end of the alignment sleeve 100, the front end of the innerferrule front section 102 are and the first fiber end 101 a are alignedin a plane perpendicular to the ferrule structure center axis.

The inner ferrule rear section 103 is anchored to the first fiber endsection 101 in an anchor length section 103 b, and from a point 101 c ofthe PCF and to the first fiber end 101 a, the PCF is free of polymercoating.

The ferrule structure comprises a hermetic solder element 86 a arrangedto surround the first fiber end section 101 to form an annular hermeticseal 106 a between the first fiber end section 101 and the inner ferrulerear section 103.

The outer ferrule rear section 105 is fixed to each of the inner ferrulefront section 102 and the inner ferrule rear section 103 to hold them ina fixed position relative to each other and to form a gap there between.The outer ferrule rear section 105 is fixed by solder 106 at each of itsends to the respective sections 102, 103 of the inner ferrulearrangement and the outer ferrule front section 105 a is fixed by solder106 to the inner ferrule front section 102.

The ferrule structure further comprises an end cap 107 arranged in frontof the first fiber end 101 c. The end cap 107 is mounted with a distanceto the inner ferrule front section 102. The end cap is fixed to theouter ferrule front section 105 a of the outer ferrule arrangement.

The ferrule structure comprises an outer alignment jacket 109surrounding the outer ferrule arrangement 105, 105 a, the outeralignment jacket preferably comprises means 109 a, 109 b for alignment.

A sensor 108 in mounted to the outer alignment jacket 109 for monitoringthe connector performance and/or for monitoring fiber damage.

The PCF assembly of FIG. 13a and FIG. 13b comprises a photonic crystalfiber (PCF) assembly PCF with a first fiber end section 111 assembledwith a ferrule structure comprising an inner ferrule arrangementcomprising an inner ferrule front section 112 proximally to the firstfiber end and an inner ferrule rear section 113 distally to first fiberend and surrounding the first fiber end section 111.

The assembly further comprises an alignment sleeve 110 arranged betweenthe inner ferrule front section 112 and the first fiber 111 end sectionto fully surround the first fiber end section 111, such that the innerferrule front section 112 supports the first fiber end section 111proximally to the first fiber end 111 a via the alignment sleeve 110.The alignment sleeve 100 supports the first fiber end section 111, bybeing collapsed in a supporting length section 110 b onto the firstfiber end section 111. In the not collapsed part 110 a of the alignmentsleeve 110 the alignment sleeve 110 has an outer diameter correlated tothe inner diameter of the inner ferrule front section 112.

The inner ferrule rear section 113 is anchored to the first fiber endsection 111 in an anchor length section 113 b, and from a point 111 c ofthe PCF and to the first fiber end 111 a, the PCF is free of polymercoating.

The ferrule structure comprises a hermetic solder element 86 a arrangedto surround the first fiber end section 111 to form an annular hermeticseal 116 a between the first fiber end section 111 and the inner ferrulerear section 113.

The outer ferrule rear section 115 is fixed to each of the inner ferrulefront section 112 and the inner ferrule rear section 113 to hold them ina fixed position relative to each other and to form a gap there between.The outer ferrule rear section 115 is fixed by solder 116 at each of itsends to the respective sections 112, 113 of the inner ferrulearrangement and the outer ferrule front section 115 a is fixed by solder116 to the inner ferrule front section 112.

The ferrule structure further comprises an end cap 117 mounted with adistance to the inner ferrule front section 112. The end cap is fixed tothe outer ferrule front section 115 a of the outer ferrule arrangement.

The ferrule structure comprises an outer alignment jacket 119surrounding the outer ferrule arrangement 115, 115 a, the outeralignment jacket preferably comprises means 119 a, 119 b for alignment.

The PCF end section shown in FIG. 14a is a hollow core PCF and comprisesa hollow core 121 and a surrounding cladding with a plurality ofcladding holes 122. At the first fiber end 123 the PCF has a metallic oranti-reflex coating on fiber facet (fiber end), for protecting againstincident light and/or back reflections.

The PCF end section shown in FIG. 14b is a hollow core PCF and comprisesa hollow core 131 and a surrounding cladding with a plurality ofcladding holes 132. At a short end part 135 adjacent to the first fiberend 133—e.g. an end part with a length l of up to about 2 mm inlength—the PCF cladding holes have been collapsed or sealed in otherways. The hollow core 131 is not sealed.

FIG. 14c shows another view of the hollow core PCF shown in FIG. 14b .Due to the sealed cladding holes 133 it can be seen that the lighttransmitted in the PCF is not fully confined in the end part with thelength l and the light is spreading out in a cone shape 134. Thealleviate this, a lens may e.g. be arranged in front of the first fiberend 133.

In FIG. 14e the hollow core PCF shown in FIG. 14c has further beenprovided with a metallic coating 133 a for protecting against incidentlight and/or back reflections.

The laser system shown in FIG. 15 comprises a laser light source 141 anda fiber delivery cable 142 for delivering light from the laser lightsource 141 to a user apparatus 144. The fiber delivery cable 142comprises as its waveguide a hollow core PCF as described above with oneor more low loss transmission bands correlated to the user apparatus. Asindicated the fiber delivery cable 142 may be rather long while stillbeing able to deliver single mode light with high efficiency and lowloss in the fundamental mode to the user apparatus 144. The fiberdelivery cable 142 has a first end 143 a and a second end 143 b. In theshown embodiment each of the first end 143 a and a second end 143 b aremounted in a ferrule structure as described above for connectingrespectively to the user apparatus 144 and the laser light source 141.

The apparatus of FIG. 16 comprises the laser system of FIG. 15 connectedto the user apparatus 114.

The apparatus of FIGS. 17a and 17b comprises a laser light source 151delivering pulsed light and a cable 152 of a supercontinuum generatingPCF arranged for generating and delivering supercontinuum light to auser apparatus 154. The fiber delivery cable 152 has a first end 153 aand a second 153 b. In the shown emobdiment each of the first end 153 aand a second end 153 b are mounted in a ferrule structure as describedabove for connecting repsectively to the user apparatus 154 and thelaser light source 151. The fiber delivery cable 152 comprises as itswaveguide a solid core PCF as shown in FIG. 17b comprising a pluralityof microstructures 166 surrounding the solid core 155.

The invention claimed is:
 1. A photonic crystal fiber (PCF) assemblycomprising a PCF and at least one ferrule structure, said PCF has acenter axis and comprises a core region and a cladding region and afirst fiber end section with a first fiber end, said ferrule structurehas a center axis and is mounted to said first fiber end section, saidferrule structure comprises an inner ferrule arrangement and an outerferrule arrangement surrounding the first fiber end section, said innerferrule arrangement comprises an inner ferrule front section proximallyto said first fiber end and an inner ferrule rear section distally tosaid first fiber end, each of said inner ferrule sections have an innerdiameter and in at least a length thereof fully surrounds the PCF, saidinner ferrule rear section is anchored in an anchor length section tosaid first fiber end section and said inner ferrule front sectionsupports said first fiber end section proximally to said first fiberend, wherein said first fiber end section has at least one mode stripperlength section and at least one of said inner ferrule front section andsaid inner ferrule rear section has a carving exposing said modestripper length section of said PCF.
 2. The PCF assembly of claim 1,wherein said carving partly surrounds the fiber.
 3. The PCF assembly ofclaim 1, wherein said mode stripper length section comprises a modestripping high index material and/or a scattering layer applied incontact with the optical fiber at said mode stripper length sectionand/or said fiber in said mode stripper length section has a roughnessRa value of at least about 0.1 μm.
 4. The PCF assembly of claim 1,wherein said assembly further comprises an alignment sleeve arrangedbetween the inner ferrule front section and the first fiber end section,such that the inner ferrule front section supports said first fiber endsection proximally to said first fiber end via said alignment sleeve. 5.The PCF assembly of claim 4, wherein said inner ferrule front sectionsupports said first fiber end section via said alignment sleeve andmechanically holds said alignment sleeve in axial position.
 6. The PCFassembly of claim 4, wherein said alignment sleeve has a length in theaxial direction which is up to the length of the inner ferrule frontsection.
 7. The PCF assembly of claim 4, wherein said alignment sleevesupports said first fiber end section, by having an inner diameter atleast along a support section thereof which is up to about 0.1 mm largerthan the PCF diameter.
 8. The PCF assembly of claim 4, wherein saidalignment sleeve is of glass.
 9. The PCF assembly of claim 4, whereinsaid alignment sleeve is of down doped silica.
 10. The PCF assembly ofclaim 4, wherein said alignment sleeve has a refractive index which isless than an effective refractive index of the cladding region.
 11. ThePCF assembly of claim 1, wherein said inner ferrule front section andsaid inner ferrule rear section are not directly fixed to each other.12. The PCF assembly of claim 1, wherein said ferrule structurecomprises a hermetic solder element arranged to surround said firstfiber end section to form an annular hermetic seal between the firstfiber end section and said inner ferrule rear section, said hermeticsolder element is arranged closer to the inner ferrule front sectionthan the anchor length section of the inner ferrule rear section. 13.The PCF assembly of claim 1, wherein said PCF is free of polymer coatingin the first fiber end section from the anchoring section of the innerferrule rear section to the first fiber end.
 14. The PCF assembly ofclaim 1, wherein said inner ferrule front section is of an at leastpartly transparent material at a wavelength between about 200 nm andabout 4 μm.
 15. The PCF assembly of claim 1, wherein said inner ferrulerear section is of fused or crystallized quartz or of a metal or alloy.16. The PCF assembly of claim 1, wherein said outer ferrule arrangementis fixed to each of said inner ferrule front section and said innerferrule rear section of said inner ferrule arrangement to hold them in afixed position relative to each other.
 17. The PCF assembly of claim 1,wherein said first fiber end section is mounted in said ferrulestructure without any direct bonds to the fiber beyond one or more bondsto the inner ferrule rear section including the anchoring.
 18. The PCFassembly of claim 1, wherein said ferrule structure comprises an end caparranged in front of said first fiber end, said end cap being mountedwith a distance or without a distance to the inner ferrule frontsection.
 19. The PCF assembly of claim 18, wherein said end cap is fixedto an outer ferrule front section of the outer ferrule arrangement. 20.The PCF assembly of claim 1, wherein said PCF has a core diameter ofless than 100 μm.
 21. The PCF assembly of claim 18, wherein the PCF is ahollow core fiber and said end cap is fixed to an outer ferrule frontsection of the outer ferrule arrangement to provide an end cap spacebetween said end cap and said inner ferrule front section, andoptionally said hollow core has a collapsed end part and a metallic oranti-reflex coating on a fiber facet.
 22. The PCF assembly of claim 21,wherein the inner ferrule arrangement comprises a passage into said endcap space for injecting and/or withdrawing fluids.
 23. The PCF assemblyof claim 1, wherein said first fiber end section has an further modestripper length section positioned between said inner ferrule frontsection and said inner ferrule rear section.
 24. The PCF assembly ofclaim 1, wherein the ferrule structure comprises a mode stripper coatingarranged in direct contact with an outer surface of the inner ferrulefront section.
 25. A laser system comprising a PCF assembly according toclaim 1, wherein said laser system comprises a laser light source, saidPCF assembly being optically connected to said laser light source forreceiving light from said laser light source and being adapted fordelivering said light to a light employing station of an apparatus.